Raltegravir is chemically known as N-(4-fluorobenzyl)-5-hydroxy-1-methyl-2-(1-methyl-1-{[(5-methyl-1,3,4-oxadiazol-2-yl)carbonyl]amino}ethyl)-6-oxo-1,6-dihydropyrimidine-4-carboxamide. Raltegravir is a potent HIV integrase inhibitor which is used for treatment of HIV infections, AIDS, and Aids Related Complex (ARC). The potassium salt of raltegravir, represented by the structural formula provided below, is marketed in USA under the trade name Isentress® by Merck & Co.

Raltegravir is generically and specifically disclosed in U.S. Pat. No. 7,169,780 B2 and potassium salt of raltegravir is specifically described by U.S. Pat. No. 7,754,731 B2. Raltegravir exhibits poor aqueous solubility where as the potassium salt of raltegravir is significantly more soluble in water and exhibit improved pharmacokinetics in animal models over raltegravir. On the contrary attempts to prepare crystalline sodium salt of raltegravir failed and resulted in an amorphous material.
Extensive study is carried out in pharmaceutical industry for development of different new salts and polymorphs of various drug substances, to obtain suitable salts and forms that possess improved performance characteristics such as aqueous solubility, improved bioavailability, chemical stability, shelf life etc.
Literature survey reveals that raltegravir potassium can exist in different polymorphic forms, which differ from each other in terms of stability, physical properties and pharmacokinetics. Very few documents in prior art are directed towards salts and polymorphs of raltegravir, which are incorporated here by way of reference.
The PCT application WO 2006/060712 A2 discloses two anhydrous crystalline forms of raltegravir potassium viz., form 1 and form 3 and one crystalline hydrate designated as form 2. Form 1 is especially known to exhibit superior bioavailability and improved pharmacokinetics over raltegravir. It can be prepared by crystallization of raltegravir potassium from a mixture of potassium base, raltegravir, water and an alcohol.
Hydrated crystalline form 2 is prepared by sonicating a mixture of raltegravir, KOH, acetone and trace amount of water whereas anhydrous crystalline form 3 is obtained by crystallization of amorphous raltegravir potassium from ethanol.
The PCT application WO 2010/140156 A2 describes amorphous form and crystalline form H1 of raltegravir potassium. The process for preparation of crystalline form H1 comprises of providing a solution of raltegravir potassium in dimethyl formamide, dimethyl acetamide or mixtures thereof and further separating and isolating the solid obtained. The amorphous form is obtained by freeze drying the aqueous solution of raltegravir potassium at −180° C.
WO 2011/024192 A2 describes novel crystalline form A, form B and amorphous form of raltegravir and processes for preparing them.
Salts often improve physical and biological characteristics of mother compounds without modifying primary pharmacological activity, based on mechanism of action. Thus there is a continuing need to obtain new salts of raltegravir having improved physical and/or chemical properties. The present invention satisfies this need by providing new salts of raltegravir with enhanced solubility in water or aqueous media as an essential property of active pharmaceutical ingredients determining the performance of pharmaceutical formulation.